Fabrication of GaN Based Light-Emitting Diodes (LEDs) with Specific Side Wall Structures

  • 陳 君彥

Student thesis: Master's Thesis

Abstract

In this thesis to enhance light extraction efficiency (LEE) a series of GaN-based light-emitting diodes (LEDs) with specific sidewall structures are fabricated and studied The device fabrication process and nanomaterials applications including fabrication of specific sidewall structures GaN-based LEDs with a nanoscale textured backside reflector fabricated via rapid convection deposition (RCD) GaN-based with hybrid structure of textured sidewalls and a nanoscale textured backside reflector and GaN-based LEDs with textured sidewalls a nanoscale textured backside reflector and a passivation layer are proposed to improve wall-plug efficiency (WPE) The optical and electrical properties of these GaN-based LEDs are studied and discussed In addition the fabrication of various combined textured sidewalls structures is discussed in detail GaN-based LEDs with textured sidewall structures fabricated via inductively coupled plasma (ICP) etching process are studied The textured sidewall structures improve sidewall roughness without degrading electrical properties Thus the total internal reflection (TIR) is effectively reduced and the light scattering probability is increased Compared with a conventional GaN-based LED at 350 mA the studied device exhibits 18 3% and 18 4% enhancements in light output power (LOP) and luminous ?ux respectively Therefore performance can be improved by employing textured sidewall structures A monolayer of self-assembled pseudo-hexagonal close-packing SiO2 nanospheres was coated onto the backside of a sapphire substrate as a hard mask Using a dry ICP etching process hemispherical patterns were transferred onto the backside of the sapphire substrate Then an aluminum (Al) metal mirror was deposited using a thermal evaporator Due to the presence of textured hemispherical patterns downward photons emitted from the active region toward the textured backside reflector reflected and scattered for light extraction rather than being absorbed by the package metal Compared with a conventional GaN-based LED at 350 mA the studied device exhibits 30 7% and 32 0% enhancements in LOP and luminous ?ux respectively In addition textured sidewall structures were combined with the hemispherical pattern backside reflector This combination improved the roughness of sidewalls and the sapphire substrate Thus the TIR is effectively reduced Downward photons emitted from the active region are scattered and redirected in arbitrary directions for light extraction The probability of light scattering is also increased Compared with a conventional GaN-based LED at 350 mA the studied device exhibits 55 8% and 49 3% improvements in LOP and luminous ?ux respectively without degrading electrical properties Finally the combination of a hybrid SiO2 microsphere (MS) antireflection passivation layer deposited on the textured sidewall structures and a hemispherical backside reflector fabricated via RCD was studied The hybrid SiO2 MS antireflection passivation layer further improved surface roughness and light scattering without degrading electrical properties The enhanced light extraction of GaN-based LEDs can be attributed to a graded-refractive-index structure Due to a reduction of TIR photons emitted from the active region more easily escape from the inside of LEDs Compared with a conventional LED at 350 mA the studied device exhibits 54 2% and 54 4% improvements in LOP and external quantum efficiency (EQE) respectively The leakage current is also reduced without degrading the electrical properties The proposed modifications improve the performance of GaN-based LEDs High-performance GaN-based LEDs are expected to compete with traditional light sources in solid-state lighting applications
Date of Award2017 Jul 3
Original languageEnglish
SupervisorWen-Chau Liu (Supervisor)

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